1. Field of the Invention
The present invention relates to an optical device that uses a light source for emitting a plurality of laser beams simultaneously and a method for controlling the optical device.
2. Description of the Related Art
Single-beam semiconductor lasers have been widely used in apparatuses for writing data to media using laser beams. Examples of such apparatuses include a laser printer, an optical disk device, and a digital copying machine.
However, in recent years, an laser diode array (LDA) capable of simultaneously emitting a plurality of beams, typically 2 to 4 beams, and a vertical cavity surface emitting laser (VCSEL) capable of simultaneously emitting approximately 40 laser beams have been developed so as to achieve higher speed and resolution. An image forming apparatuses that performs image formation at higher resolution and higher speed by employing an LDA or a VCSEL as a laser-beam light source for exposing a photosensitive element has been put to practical use.
Meanwhile, LDAs and VCSELs are more costly than single-beam semiconductor lasers. Therefore, a beam-splitting method is proposed in which a beam emitted from an LDA or a VCSEL is split into two beams using a half-mirror prism so as to illuminate photosensitive elements corresponding to two colors. Use of the beam-splitting method can reduce the number of the LDAs or VCSELs by half while keeping the same level of resolution and processing speed, thereby reducing a manufacturing cost.
Meanwhile, a laser beam used for illuminating a photosensitive element needs to emit light with a predetermined light amount. Therefore, adjustment of the light amount is performed at, for instance, an assembly plant in which image forming units including light sources are assembled. The adjustment of the light amount is generally performed by providing, for example, a photodiode (PD) that receives a laser beam to monitor a light amount on a photosensitive element, which is a surface to be scanned, of the PD. In a state where the PD is caused to emit a laser beam, the light amount is adjusted to a target light amount by manually operating a variable resistor that controls the output power of the laser beam according to the output power of the PD.
Accordingly, the greater the number of laser beams for use in exposing a photosensitive element, the longer time is required to adjust amounts of light in the assembly plant, resulting in an increase in an assembly cost. It is also required to mount as many variable resistors for adjusting light amount as the laser beams, resulting in an increase in the size of an electronic circuit.
To solve this problem, there has already been known a technology that suppresses an increase in the time needed for adjusting a light amount and an increase in a circuit size by employing an automatic adjustment of the light amount in lieu of a manual adjustment of the light amount by using variable resistors. The automatic adjustment is performed by storing in advance the output power of the PD at which the target light amount is achieved.
For example, disclosed in Japanese Patent Application Laid-open No. 2002-298354 is a technology according to which predetermined emission power of a laser beam and a monitored voltage corresponding to the emission power are stored in a memory. According to the technology disclosed in Japanese Patent Application Laid-open No. 2002-298354, a relational expression between apparatus-specific monitored voltages and laser drive currents, which are based on output power, is determined from the monitored voltages stored in the memory. Then, based on a currently monitored voltage and the relational expression, emission power is monitored and drive currents are controlled to thereby achieve reduction in a circuit size.
However, the conventional method for adjusting the light amount only performs recording adjustment values for the variable resistor as digital values, by which accuracy in the adjustment of the light amount is not improved.
Meanwhile, there is proposed a beam-splitting method that splits each laser beam into two beams using an optical splitting prism and two polygon mirrors having different angles of reflection and that causes the two laser beams to respectively illuminate two photosensitive elements used for different image-forming colors. Conventionally in this beam-splitting method, adjustment of the light amount in a writing unit that generates scanning beams by deflecting the laser beams has been performed for each of the photosensitive elements corresponding to two colors. The number of adjustments to be performed in this case is (the number of the laser beams)×(two(=the number of colors)), amounting to twice that required for a normal method that does not employ the beam-splitting method. Furthermore, when the beam-splitting method is employed, a storage medium having a larger storage capacity is required because data corresponding to (the number of the laser beams)×(two(=the number of colors)) is to be stored in the storage medium.
FIG. 14 illustrates an example of adjustment data for a writing unit that scans one photosensitive element without performing laser-beam splitting using a 40-channel VCSEL capable of emitting 40 laser beams at a time. In this case, pairs of exposure power and monitored voltages for the 40 channels are stored in a storage medium. FIG. 15 illustrates an example of adjustment data for a writing unit that uses a VCSEL identical to that illustrated in FIG. 14 but splits each of the laser beams of the 40 channels into two beams by the beam-splitting method so as to scan each of photosensitive elements corresponding to two colors. When the beam-splitting method is used, pairs of exposure power and monitored voltages for the two colors are stored in a storage medium in this way.
In the light amount adjusting process performed in the writing unit, the light amount of a laser beam is measured when the laser beam reaches a position of the photosensitive element via optical components including a plurality of lenses and a polygon mirror. Accordingly, the light amount of the laser beam on the photosensitive element is considerably minute as compared with the light amount of the laser beam immediately after the emission.
When the number of laser beams is increased, the light amount per laser beam becomes more minute (a few to ten-plus microwatts) and more sensitive to a noise, making it difficult to obtain satisfactory adjustment accuracy. As a result, there has been a problem that a large difference in the amounts of light develops among laser beams during scanning of a photosensitive element even after the adjustment of the amounts of light has been performed, thereby causing uneven concentration in an image.
Thus, when a laser light source capable of simultaneously emitting a plurality of laser beams is used, there is a need for increasing accuracy in adjusting the amounts of light of the laser beams emitted from the laser light source.